High Diversity, Low Disparity and Small Body Size in Plesiosaurs (Reptilia, Sauropterygia) from the Triassic–Jurassic Boundary

Abstract

Invasion of the open ocean by tetrapods represents a major evolutionary transition that occurred independently in cetaceans, mosasauroids, chelonioids (sea turtles), ichthyosaurs and plesiosaurs. Plesiosaurian reptiles invaded pelagic ocean environments immediately following the Late Triassic extinctions. This diversification is recorded by three intensively-sampled European fossil faunas, spanning 20 million years (Ma). These provide an unparalleled opportunity to document changes in key macroevolutionary parameters associated with secondary adaptation to pelagic life in tetrapods. A comprehensive assessment focuses on the oldest fauna, from the Blue Lias Formation of Street, and nearby localities, in Somerset, UK (Earliest Jurassic: 200 Ma), identifying three new species representing two small-bodied rhomaleosaurids (Stratesaurus taylori gen et sp. nov.; Avalonnectes arturi gen. et sp. nov) and the most basal plesiosauroid, Eoplesiosaurus antiquior gen. et sp. nov. The initial radiation of plesiosaurs was characterised by high, but short-lived, diversity of an archaic clade, Rhomaleosauridae. Representatives of this initial radiation were replaced by derived, neoplesiosaurian plesiosaurs at small-medium body sizes during a more gradual accumulation of morphological disparity. This gradualistic modality suggests that adaptive radiations within tetrapod subclades are not always characterised by the initially high levels of disparity observed in the Paleozoic origins of major metazoan body plans, or in the origin of tetrapods. High rhomaleosaurid diversity immediately following the Triassic-Jurassic boundary supports the gradual model of Late Triassic extinctions, mostly predating the boundary itself. Increase in both maximum and minimum body length early in plesiosaurian history suggests a driven evolutionary trend. However, Maximum-likelihood models suggest only passive expansion into higher body size categories.

Ichthyosaurs from the French Rhaetian indicate a severe turnover across the Triassic–Jurassic boundary

“…Hallam and Wignall 1997;Wignall and Bond 2008;Smith et al 2014). But other groups such as plesiosaurs and neoichthyosaurians radiated immediately after (Benson et al 2012;Fischer et al 2013), implying a Rhaetian or older divergence of lineages. Under this revised scheme, the Rhaetian and the Triassic-Jurassic boundary therefore concentrate a massive faunal turnover for marine reptiles.…”

“…The recent discovery of a possible non-parvipelvian ichthyosaur in the earliest Jurassic of Wales (Martin et al In Press) indicates that our understanding of the tempo and amplitude of these events remain largely unknown, as is their synchronicity with other biotic and abiotic events (Benson et al 2012).…”

New insights on the systematics, palaeoecology and palaeobiology of a plesiosaurian with soft tissue preservation from the Toarcian of Holzmaden, Germany

“…They are double-headed in most of the centra, as is common in early plesiosaurians (Benson et al 2015), but the rib head is single in the posterior part of the neck (C37-C40; Fig. 2c) and thus differs from that reported for the genus Microcleidus which exhibits widely separated posterior cervical rib facets (diagnosis of Benson et al 2012). The diapophysis and parapophysis are identical in length and shape in the anterior half of the neck (C1-C20), and are closely spaced so that there is no horizontal groove between the two facets.…”

Reassessment of 'Plesiosaurus' megacephalus (Sauropterygia: Plesiosauria) from the Triassic-Jurassic boundary, UK

“…BRSMG Cb 2335 was from the lowermost beds of the Blue Lias Formation (Lower Lias, Hettangian) of Street-on-the-Fosse (Wright, 1860, p.391), a village about 14 km ENE of Street, Somerset, UK (Cruickshank, 1994a;Storrs and Taylor, 1996). Specimens in and around Street probably originate from the pre-Planorbis beds (below the first occurrence of Psiloceras planorbis), in the Psiloceras tilmanni Zone that immediately follows the Triassic-Jurassic boundary, however, it is possible that some specimens are from slightly younger deposits (Storrs and Taylor, 1996;Benson et al, 2012). Swinton (1930Swinton ( , 1948 wrongly gave the locality of BRSMG Cb 2335 as Street and ascribed the occurrence to the "?angulatum Zone" (upper Hettangian) (see discussion in Cruickshank 4 FIGURE 2.…”

“…Referred specimens. The following specimens are referred to Atychodracon megacephalus pending a thorough evaluation of the material with reference to the holotype: LEICS G221.1851 (the neotype specimen) from Barrow-upon-Soar, Leicestershire, UK (Cruickshank, 1994a(Cruickshank, , 1994b; NMING F10194, a partial skeleton including the skull (but no mandible) from Street (Smith, 2007;Benson et al, 2012); NMING F8749, a partial skeleton including a damaged skull and suffering from pyrite decay, also from Barrow-upon-Soar (Smith, 2007). A complete skeleton from Wilmcote, Warwickshire, UK, sometimes referred to 'P.'…”